Single-mode dielectric filters are in widespread use in many communications systems, including both low- and high-power use within the cellular communications industry. In particular, duplex filters, used in many handsets will typically employ this form of filter technology and some higher power applications exist, although the high losses associated with commercial products typically restrict their use to power levels of a few watts (mean) or less.
Interest in the use of multi-mode filters is growing, since these filters allow the same piece of dielectric material (or ‘puck’) to be, effectively, re-used multiple times, to form a more complex filter characteristic. This will have, typically, a steeper roll-off and a wider pass-band bandwidth than an equivalent single-mode resonator could achieve. It will also, typically, result in lower losses, due to the reduction in the number of times the signal needs to be coupled into and out of the dielectric material. A typical example would be a triple mode filter, in which the dielectric material is excited in three dimensions or ‘planes’—the X-plane, the Y-plane and the Z-plane. The excitation can be in the form of H-field (magnetic) or E-field (electric) or a combination of the two (in any ratio).
The structure (whether multi-mode or single-mode) is that of a cavity filter. A piece of dielectric material (puck) is coated with conductive material with the exception of at least one aperture which allows the unfiltered signal to be input to the dielectric material, and the filtered signal to be output from the dielectric material. This is a widely-used and inherently low loss structure. A cavity resonator spreads the current out evenly over the whole surface and so minimises the current concentration over that surface. By contrast, a combline filter, for example, concentrates the current on the central rod, so the current is not evenly distributed and hence the filter has generally higher losses.
In order to achieve a steep roll-off, together with a wide pass-band bandwidth, it may be desirable to cascade a plurality of resonators in series. This process will typically result in a significant increase in the loss in the (wanted) pass-band, due to both the insertion loss of the dielectric material itself (i.e. the dielectric losses within that material) and the coupling losses in transferring energy into and out of the dielectric.
In practice, however, the use of multiple resonators connected in series raises difficulties. For example, resonators may be coupled together by placing an aperture in the conductive coating of one resonator next to a corresponding aperture in the coating of an adjacent resonator. Gaps between resonators are inevitable in a practical multi-resonator filter, due to imperfections in the uniformity of the conductive coating (for example) surrounding the resonators, together with the basic thickness of that coating. The coatings of adjacent resonators will touch at locations where they are thickest, while gaps will be formed where the coatings are thinner. These gaps, together with the intrinsic thickness of the silvering, create a void between the two apertures. The presence of this void has two consequences for an aperture-coupled filter:
1. The introduction of a small amount of a dielectric (air) with a very differing dielectric constant to the dielectric of the resonators, may lead to a shift in the resonant frequency of the resonators. Whilst it is theoretically possible to compensate for this shift at the design stage of the filter, its unpredictability, due to the unpredictability of the size of the gap for a given manufactured example of the filter, makes full compensation at the design stage essentially impossible. Whilst this residual, unpredictable, frequency shift may not be large in percentage terms, it can be catastrophic for a tightly-specified filter, with a narrow pass-band made up of the juxtaposition of multiple resonances. Note that in the case of a multi-mode resonator, this shift may be significantly greater for one mode than for the others, which will not only alter the overall centre frequency, but also significantly impact the filter's passband shape (e.g. ripple).
2. The very high electric field present in the small air gap is the primary source of breakdown and hence the primary limitation on the ability of a filter to handle high power signals in many designs.
A filter is desired which alleviates these and other problems.